Generally, specific cells in the body tissue can be stimulated by electrical fields acting from the outside. Nerve or muscle cells can in particular be excited by depolarization of an axon or a bundle of fibers, respectively, stimulated by an external field, and the triggering of action potentials and the subsequent stimulus conduction resulting therefrom. This is done in that the electric fields cause electrical currents in the tissue, which in turn trigger action potentials in these cells. This type of triggering action potentials by an electrical field acting directly upon the axon is a non-physiological process: In nature, action potentials are generated in the cell body of the nerve cell itself, after signals coming in via the dendrites have been linked respectively.
The electric field acting from the outside must for triggering such an action potential reach a certain temporal course and a certain minimum strength. In particular, regarding the triggering of an action potential, different cell types also react differently to temporal courses and strengths of the fields acting from the outside. By way of example, sensory nerve fibers also being responsible for the transmission of pain signals, due to their smaller diameter require a higher field strength for depolarization (i.e. for triggering an action potential) than motor nerve fibers. At moderate stimulus intensities it is therefore possible to stimulate only the motor but not the sensory fibers and therefore to stimulate nerves virtually without pain.
The principle of magnetic induction can be used in particular for this type of stimulation. In this, a time-varying magnetic field generates an induced electric field. The time-varying magnetic field can be generated by a coil which has time-varying current passing through. This coil is located, for example, on the skin above the nerve tissue to be stimulated, so that the generated magnetic field can penetrate the tissue and, according to the induction principle, generate the currents in the tissue necessary for stimulation. In this, stimulation by the so-called inductive magnetic stimulation can occur contactless, since the magnetic field can penetrate body tissue without hindrance. The time-dependent magnetic fields are generated by brief current pulses having a duration of usually of 50-400 microseconds. The principle of inductive stimulation is based principally on a temporal change of the magnetic field. In this manner, only time-varying electric fields can also be created in the tissue. Therefore, no efficient single monophasic rectangular pulses having a direct (DC) component can for example be generated as they are used in the electrical stimulation.
An advantage of the inductive magnetic stimulation is that it is contactless, as the magnetic field of the coil also reaches the body tissue being at a certain distance from the coil. Therefore, nerve cells can be stimulated in a sterile manner. Another advantage is that the method, in contrast to electrical stimulation via electrodes, is almost completely painless, because contrary to electrical stimulation, no high current densities can arise at the locations of application of the electrodes. For these reasons, the method is also particularly suitable for stimulation of deeper-lying tissue structures (e.g. the cerebral cortex through the cranial bone) and for pain-free muscle stimulation e.g. in the field of rehabilitation.
Due to these advantages, the inductive magnetic stimulation was able to already prevail over electric stimulation in some fields or even open up new fields of application. The procedure is very common for application to the central and the peripheral nervous system.
Currently it is the only non-invasive procedure, with which, for example, certain brain areas can be selectively activated without any pain for the individual (i.e. triggering nerve action potentials or subliminal influencing of nerve cells in these regions) such that responses by nerve cells can be processed by the body just like, or at least very similar to, naturally occurring nerve impulses.
The inductive magnetic stimulation is used in fundamental research as a tool for joint examination together with functional magnetic resonance imaging. Selective excitation (and inhibition) of certain brain areas can be induced via pulses, the effects of which can in turn be examined by magnetic resonance imaging.
Furthermore, there are applications of inductive magnetic stimulation regarding peripheral motor nerves. In this, repetitive continuous stimulation with fast pulse sequences (10 to 50 pulses per second) is of great significance.
Applications for apparatuses used in high performance sports are known.
FIG. 1 shows a typical arrangement of the previous use of the inductive magnetic stimulation. The pulse source 110 generates a brief strong current pulse and conducts it to the coil 120. The coil 120 is positioned close to the body nerve tissue to be stimulated, so that the generated magnetic field can penetrate this tissue structure. The magnetic field generated by the coil induces an electric field in the body tissue, presently the upper arm 130, which stimulates nerve and muscle tissue via the resulting currents.
However, for the inductive magnetic stimulation, this detour through the magnetic field of the coil also causes important technical problems:
The required magnetic flux densities are in the range of about 1 Tesla, so that during the very brief magnetic stimulation pulse, extremely high electric power must be provided to the coil in order to generate the appropriate field energies. The required electrical power can reach values of several megawatts and the currents can reach several kilo amperes at voltages of several kilovolts. Therefore, the pulse sources are technically complex; furthermore the coil very quickly overheats due to the current heat losses, where it must presently be additionally considered that the coil may not reach too high temperatures as it is a component that can directly contact the body.
In order to nevertheless be able to provide respective currents and energies for this type of stimulation with a reasonable technical available effort, magnetic stimulation devices presently operate according to the principle of the resonant oscillating circuit in which a capacitor discharges its energy into the coil. The principle of generating powerful pulses for the coil is thus based on a continuous charge of the oscillating circuit capacitor via a charging device at relatively low power and the rapid discharge of the energy content of this capacitor to the coil for generating the brief strong magnetic field pulse.
FIG. 2 shows the basic circuit structure of an inductive stimulation device as used in the first devices in particular for contactless stimulation of cortical nerve structures through the intact cranial bone (R. Siebner, U. Ziemann, “Das TMS-Buch/The TMS-book”, Springer publishing house, ISBN-13 978-3-540-71904-5). For this, the circuit uses a powerful damped electrical oscillation circuit (resonator) comprising a capacitor 220, a damping resistor 230, a diode 240, a thyristor 250 and the coil 260. The charging circuit 210 charges the capacitor 220 to a voltage of several thousand volts. The energy content of the capacitor amounts to several 100 joules. The thyristor 250 serves as a switch which during ignition connects the capacitor 220 with the magnetic coil 260 and thus lets the current flow in the coil begin.
FIG. 3 shows the temporal course of current and voltage in the coil according to the circuit of FIG. 2. Upon ignition of the thyristor, an initially sinusoidally increasing current flow develops, which generates a corresponding magnetic field increasing with time. This magnetic field in turn induces ring currents in the body tissues as a result of its change over time. The phase-shifted coil voltage has its first zero crossover exactly upon reaching the current peak value. Since from this point on, the coil voltage reverses its sign, the damping circuit comprising the resistor 230 and the diode 240 now becomes active, preventing further oscillation of the oscillating circuit. Therefore, the coil current, after reaching its peak value, slowly falls back to zero. The typical time period between the thyristor ignition and reaching the current peak value is about 50 to 150 microseconds. By means of this damping circuit, however, the entire pulse energy of the capacitor in the resistor 230 and in the coil conductors of the coil is transformed to heat.
This damping circuit being employed in the first devices, which dampens the oscillation from the first dropping current edge (after one quarter of the period duration), characterizes the so-called monophasic stimulation, as the coil current during the pulse only flows in one direction, i.e., does not change its sign. Since for these devices, the pulse energy of the magnetic field is completely lost with each pulse, these devices have particularly high energy consumption.
These first devices were therefore not suitable for so-called repetitive stimulation for which 10 to 50 pulses per second are required. Furthermore, also the size of the devices and their high price make it difficult to open new fields of application.
Therefore, the most important development goal for the devices for inductive magnetic stimulation lies in the reduction of energy consumption and heating of the coil (R. Siebner, U. Ziemann, “Das TMS-Buch”, Springer publishing house, ISBN-13 978-3-540-71904-5). It was shown by experimental studies, that an undamped sinusoidal temporal course of the coil current and thus also of the magnetic field at the same amplitude shows an equivalent effect regarding nerve stimulation as the current profile of FIG. 3.
FIG. 4 shows the basic circuit configuration of another known stimulation device, as used in a later generation of devices. This device generates sinusoidal current or field pulses, respectively. Here as well, the charging circuit 210 charges the capacitor 220 to a voltage of several thousand volts. The thyristor 410 again serves as a switch which during ignition connects the capacitor 220 with the magnetic coil 260. In contrast to the monophasic stimulator circuit of FIG. 2, however, no damping circuit is used for this circuit, so that the oscillating circuit continues to oscillate even after the first zero crossover of the coil current.
FIG. 5 shows the temporal course of current and voltage in the coil according to the circuit of FIG. 4. Upon igniting the thyristor, a sinusoidally increasing current flow develops, which generates a corresponding magnetic field increasing with time. After half a sinusoidal oscillation, at time T/2, the current in the oscillating circuit changes its polarization. At this time, the diode 420 takes over conduction of the coil current until a full sinusoidal oscillation at time T is reached. A renewed reversal of the current direction and thus continued oscillation is prevented because the thyristor 410 is at this time T no longer conductive. Due to the reversal of the direction of current during a pulse at time T/2, this type of stimulation is generally referred to as biphasic magnetic stimulation.
It can be achieved by the circuit principle according to FIG. 4, that a large proportion of the field energy expended for the coil 260 can be returned to the capacitor 220 thus reducing the losses in both the pulse source as well as in the coil 260. The losses of the circuit of FIG. 4 mainly result via the ohmic resistances of the circuit components involved and their connection cables.
As, however, the current amplitude, required for successful stimulation, remains approximately unchanged compared with the devices with monophasic pulse shape, the necessary voltage and the energy content of the capacitor 220 remain nearly the same as with monophasic devices.
FIG. 6 shows a further development of the known circuits of inductive motors magnetic stimulators, as was used in a later generation of devices (R. Siebner, U. Ziemann, “Das TMS-Buch”, Springer publishing house, ISBN 13978-3-540-71904-5) Here as well, the charging circuit 210 charges the capacitor 220 to a voltage of several thousand volts. The thyristor 610 again serves as a switch which during ignition connects the capacitor 220 with the magnetic coil 260.
FIG. 7 shows the temporal course of current and voltage in the coil according to the circuit of FIG. 6. Upon igniting the thyristor, a sinusoidally increasing current flow develops, which generates a corresponding magnetic field increasing with time. After half a sinusoidal oscillation, at time T/2, the current in the oscillating circuit reaches its first zero point. If at this point in time the second thyristor 620 is not ignited, reversal of the current direction is not possible, so that a continued oscillation is prevented already after half a wave. Ignition of the thyristor 620 at a later time generates a further half-wave pulse in the coil with reversed current and magnetic field direction. Alternatively, however, upon reaching the first current zero point, the second thyristor 620 can also be ignited directly so that a full sinusoidal oscillation is formed, similar to FIG. 5. In any case, also for this circuit, the field energy of the coil is to a large extent returned to the capacitor.
Depending on the choice of the end time of the pulse, it is therefore distinguished regarding the pulse shape of the inductive stimulation devices between biphasic full-wave stimulation (duration of the current pulse a full sine period) and biphasic half-wave stimulation. It is disadvantageous with the biphasic half-wave stimulation, however, that after the pulse, the voltage direction in the capacitor is inverted compared with the state prior to the pulse discharge, making the respective charging circuit more complex. Furthermore, in the biphasic half-wave stimulation, the direction of the magnetic field also changes, so that successive pulses create slightly different effects in the tissue.
The energy recovery in accordance with the circuits of FIG. 4 and FIG. 6 allows a reduction of the energy lost with each pulse and thus also of the power heat losses in the coil and the power electronics. This also allows the construction of repetitive inductive stimulation device, which can deliver up to 100 pulses per second. However, especially for this repetitive operation, energy consumption and coil heating is still considerable. In particular coil heating results from the very high coil currents required, being in the kilo ampere range.
Another way to reduce energy losses can be achieved by reducing current heat losses of the coil (R. Siebner, U. Ziemann, “Das TMS-Buch”, Springer publishing house, ISBN-13 978-3-540-71904-5). This is done by increasing the effective conductor cross-section, in that, on the one hand, thicker conductor material can be used and, on the other hand, the conductor can be filamented with high-frequency wire, so that the current displacement effects in the conductor are reduced. However, the electrical resistance of the coil cannot be reduced arbitrarily for weight reasons.
As to the temporal course of the stimulus pulse, the three wave types mentioned, the damped monophasic pulse, the biphasic half-wave pulse and the biphasic full-wave pulse, still represent the only pulse shapes that are used in commercial inductive magnetic stimulation devices. All these wave shapes are ultimately based on the principle of the resonant oscillating circuit, wherein the coil is the inductor.
Therefore, the previously used devices also have the great disadvantage that the pulse duration depends on the inductance of the coil. In particular, for example, small coils often have design-related lower inductance than large coils; therefore, the pulse duration with previous systems could not be kept constant in an optimal range when using different coils.
Occasional experiments with other pulse shapes, as in Peterchev et al. 2008 with a rectangular shape (A. V. Peterchev, R. Jalinous, and S. H. Lisanby: A Transcranial Magnetic Stimulator Inducing Near-Rectangular Pulses With Controllable Pulse Width (cTMS), IEEE Transactions on Biomedical Engineering, vol. 55, no. 1, 2008) are either very energy inefficient or they lead to highly complex technical structures and are therefore too expensive for commercial technical realization
For all applications, the disadvantage, therefore, of inductive magnetic stimulation still is high energy consumption, very rapid overheating of the coil and high weight of the charging and pulse generating electronics.
Another disadvantage is that the temporal course of the stimulus pulse can not be individually flexibly adapted to certain nerve cell or axon types or other requirements.